Most home builders have good intentions when they provide rough plumbing layouts in basements. This is both good and bad. It is good that you don’t have to put the expense of tearing up concrete to put in your own plumbing, etc. Most fixtures have been considered and can be worked into the space they have provided. However, this also limits the layout of the entire basement. You have to put the bathroom right where they have chosen and hope you can be happy with the configuration of rooms that will fit around it. Even the bathroom can only be configured the way the rough plumbing is laid out. The location of the door is usually implied as well, so all areas are predesigned in a sense because of the plumbing configuration. The savings of not having to put in your own plumbing, etc. could be worth the limited design potential.
If you don’t have rough plumbing provided, this too is a blessing and a curse. If you are simply designing an additional living room there may not be a need for a bathroom or plumbing. In most cases, if someone is planning to make use of their basement, they usually have additional bedrooms, fitness rooms, theaters or other spaces in mind. For convenience (especially with bedrooms) providing a bathroom is essential. This means you must cut through existing concrete (unless you are fortunate to have a framed floor over crawl space) and run new lines (supply and waste) to the new locations. Most basements will have floor drains provided and the water heater is usually located in the basement. This is an advantage which can be utilized, and if planning is done correctly near existing plumbing supply lines, the excavation, demolition and repair will be minimal.
So the long and short is that there are advantages and disadvantages to having or not having rough plumbing provided. In either case, a good designer can take advantage of the situation and give you the best solution to fit your needs and the space provided. Don’t be afraid to explore the options – paper is cheap – so find the design that you will love!